Pyrophosphate, especially melamine pyrophosphate is a compound obtained by binding pyrophosphoric acid, which is a condensed phosphoric acid, with melamine, and is a useful material as a flame retardant which is added to paint, synthetic resin or the like. Therefore, various kinds of methods for producing melamine pyrophosphate have been proposed.
For example, a producing method is disclosed wherein melamine is mixed with hydrochloric acid to make melamine hydrochloride salt in aqueous solution, then sodium pyrophosphate is added to precipitate the melamine pyrophosphate (Patent document 1). However, this method had a defect that a producing cost was high, since the expensive pyrophosphate should be used as a raw material, and a washing process by water and a filtration process were required in order to remove the halogen.
In addition, a producing method of melamine pyrophosphate is disclosed wherein pyrophosphoric acid is reacted with melamine at 0° C.-60° C. in aqueous solution (Patent document 2). However, in this case there was also a defect that producing cost was high, since expensive pyrophosphate should be used as a raw material, and a filtration process was required.
Furthermore, when pyrophosphate such as melamine pyrophosphate is manufactured by the dehydration condensation reaction, in some cases, not only pyrophosphate but also triphosphate and polyphosphate or the like are produced due to an overreaction. Therefore, there was a defect in this case that a degree of purity of pyrophosphate declines.